On-device client API (communication between UI and client)

This epic has been created following the "Improving OTA updates for UI based devices" session on the Mender planning in May 2020.

There are more details in the document, but the most important conclusions are:

• We focus on providing local (non-networked) API first, so that UIs can use it
• Most likely candidate for protocol is D-Bus, but HTTP should also be investigated
• Networked API can be built on top later (e.g. an MQTT daemon that talks D-Bus to Mender on one side, and MQTT on the network side)
• We will provide a notification API and an action API
  • Important that download progress is part of the notification API
• Authentication to the API is not decided, needs research

This translates roughly into Acceptance criteria, but it will probably have to be refined a little bit as part of planning this epic.